Below is an exert from the api help.
"If you are using the Name parameter, then specify the XYZ coordinates in terms of the context where the item was created. For example, if you want to select a sketch point using its name (for example, "Point1") in the Name argument, then specify XYZ in terms of the sketch where the point was created. Even if the sketch is not active, specify the XYZ values in terms of sketch space when you use the Name parameter. In certain situations, you can also pass in the XYZ coordinates as simply ( 0,0,0 ). For example, to select a feature shown in the FeatureManager design tree, you do not need to specify an XYZ coordinates. However, to select objects such as notes or faces, or when you need a point location picked, you must specify the XYZ coordinates. If you are not using the Name parameter as a filter, then specify the XYZ coordinates in terms of model space. "
The best thing to do is give a name to your face. Right click on your face, click properties in the face section, and give it a name. Then use PartDoc:GetEntityByName to select your face.
And here is the code....
Dim swApp As SldWorks.SldWorks
Dim swModel As ModelDoc2
Dim swPart As PartDoc
Dim swFace As Face2
Dim swSelMgr As SelectionMgr
Dim swSelData As SelectData
Set swApp = Application.SldWorks
Set swModel = swApp.ActiveDoc
Set swPart = swModel
Set swSelMgr = swModel.SelectionManager
Set swFace = swPart.GetEntityByName("FaceForFeature", swSelectType_e.swSelFACES)
If Not swFace Is Nothing Then
swApp.SendMsgToUser "Found face"
Set swSelData = swSelMgr.CreateSelectData
swSelMgr.AddSelectionListObject swFace, swSelData
swApp.SendMsgToUser "Couldn't find face"